The present invention relates to a method of isolation of a semiconductor device.
The prior art to which the invention is directed includes the local oxidation of silicon method (hereinafter referred to as LOCOS method). The LOCOS method is a technique such as that described below.
As shown in FIG. 3A, the surface of a semiconductor 31, for example, silicon, is oxidized to form a buffer silicon oxide 32 for reducing stress generated during local oxidation, and further a silicon nitride film 33 which is to be employed as a material for an oxidation mask used in local oxidation is deposited by CVD. Next, as shown in FIG. 3B, a resist 34 is formed in a desired configuration using photoetching method, and with this resist 34 used as a mask, the silicon nitride film 33 is etched. Then, oxidation is carried out. As a result, as shown in FIG. 3C, the region which is covered with no silicon nitride film is oxidized to form a thick oxide film 35 for isolation, whereas the region which is covered with the silicon nitride film 33 is only slightly oxidized. Next, as shown in FIG. 3D, the silicon nitride film and the buffer silicon oxide are successively etched to complete an element isolation region 36 and an active region 37. Thereafter, a semiconductor element is formed in the active region 37.
In the LOCOS method, after the masking layer comprised of silicon nitride is provided on the element region, i.e., active region, the oxidation treatment is carried out, so that a field oxide film which isolates the active region is formed. Accordingly, the field oxide film 35 extends into the active region from the boundary between the element isolation region and active regions in the form of a relatively long and narrow extension (known as "bird's beak") having a length l, resulting in the active region being narrowed, as shown in FIG. 3D). The length l of the bird's beak is usually 0.5 .mu.m or more, although it depends on the oxidation conditions or the like, and the element isolation region may not be narrowed, either. To reduce the bird's beak, it may be effective practice to thin the buffer silicon oxide or thicken the silicon nitride film. However, in either case, a large stress is applied to the semiconductor substrate and a large number of crystal defects are thereby induced therein in the LOCOS process, resulting in deteriorated isolation characteristics. Accordingly, it is impossible with the conventional LOCOS to reduce the above-described bird's beak while maintaining excellent isolation characteristics.